Processes for the production of iron-nickel alloys having a high-nickel content

ABSTRACT

A process for improving the weldability of iron-nickel alloys with a nickel content higher than 30 percent in weight, which introduces into the alloy during its production an addition of at least one of the elements of the group vanadium, titanium, zirconium and niobium, said elements forming nitrides with the nitrogen present in the alloy.

United States Patent Wache Mar. 7, 1972 [54] PROCESSES FOR THE PRODUCTION OF IRON-NICKEL ALLOYS HAVING A HIGH-NICKEL CONTENT [72] Inventor: Xavier Wache, La Turlurette, Sauvignyles-Bois, (Nievre), France [22] Filed: Jan. 27, 1970 [21] Appl. No.: 6,140

Related US. Application Data [63] Continuation-impart of Ser. No. 652,484, July 11,

1967, abandoned.

[30] Foreign Application Priority Data July 12, 1966 France ..69164 [52] US. Cl. ..75/129 [51] Int. Cl. ..C22c 39/40, C22c 39/50, C22c 39/54 [58] FieldoiSeareh ..75/l23 B, 123 .1, 123 K, 123 M, 75/126 .1, 128 N, 129

[56] References Cited UNITED STATES PATENTS 1,580,662 4/1926 Girin ..'..7.75/12 3 K Ritter", 115/123 K Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Joseph E. Legru Attorney-Cameron, Kerkam 8L Sutton [57] ABSTRACT A process for improving the weldability of iron-nickel alloys with a nickel content higher than 30 percent in weight, which introduces into the alloy during its production an addition of at least one of the elements of the group vanadium, titanium, zirconium and niobium, said elements forming nitrides with the nitrogen present in the alloy.

1 Claim, 3 Drawing Figures PROCESSES FOR THE PRODUCTION OF IRON-NICKEL ALLOYS HAVING A HIGH-NICKEL CONTENT CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my application Ser. No. 652,484, filed July 11, 1967, for Iron-Nickel Alloys Having a High Nickel Content which is abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a process for forming ironnickel alloys having a high nickel content.

The storage and transport of liquefied gases, such as methane, require the construction of large containers made of materials meeting the special requirements imposed by the storage of this type of product, in particular the low temperatures, about -160 C. in the case of methane, at which liquefied gases are stored.

The principal properties which the materials used in the construction of such containers are required to have are as follows: (i) adequate mechanical strength at very low temperatures, (ii) absence of brittleness at very low temperatures, (iii) a very low coefficient of thermal expansion between room temperature and the temperature of use, (iv) good formability, and (v) good weldability by a variety of welding procedures.

One of the materials that has been acknowledged to constitute a good compromise between these various requirements is iron-nickel alloy containing 36 percent nickel and sold under the trademark Invar. This alloy has been used for the construction of tanks forming an integral part of the hull of a ship; the tanks have been tested and have given good results.

Manufacturing tests have shown that one of the difficulties in using this material is the production of good welds characterized by the absence of cracks in the welded joints, the absence of cracks in the zones of the alloy affected by heat during welding, i.e., the zones adjacent to the welded joint, and the persistence in the welded joints and the zones affected by heat of metallurgical characteristics and physical properties similar to those of the base metal.

Cracks in the welded joint may be prevented by careful choice of the tiller metal if used or by precise control of the welding conditions when filler metal is not used.

The appearance of cracks in the heat-afiected zones is more difficult to prevent. It is often due to brittleness of the base metal at temperatures higher than 500 C. During welding, the base metal is subjected to considerable stress. If the metal is ductile, this stress does not have serious consequences, but when the metal is brittle it may cause breaks. If ferro-nickel containing 36 percent nickel is prepared without special precautions, it has a hot-shortness range that is shown in the rapid-drawing test by a minimum of elongation and reduction of cross-sectional area at break. This minimum is in the temperature range of 600 to l,000 C.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the accompanying drawing shows two curves; curve A shows the percent elongation at break of ferro-nickel containing 36 percent nickel, plotted against the temperature and curb B shows the percent reduction in cross-sectional area at break for the same alloy, plotted against the temperature;

FIG. 2 shows curves similar to those of FIG. 1 curve A representing a known alloy and curve B representing an alloy of the present invention containing zirconium; and

FIG. 3 shows curves for alloys of the present invention containing titanium.

The brittleness shown by the minima of the curves of FIG. 1 at about 600 to l,000 C. may cause certain difficulties in welding during the manufacture of containers for the storage tained from tests of a drawn bar treated before the tests to hot traction by heating the bar for 1 hour at 1, 100 C. followed by cooling in air and then working. Analysis of the casting so used S 0. 084 P 0. 016 M n 0. 42 N1 35. 58 Cr 0. 05 Cu 0. 07 Mo, U Traces N 0. 0029 O 0. 0042 Fe Remainder This hot-shortness has been linked with the sulphur content of the metal when it exceeds 0.002 percent. It has been proposed to remedy this by rigorous desulphurization, for example by ,he method described in our French Pat. application PV 40,297 filed on Nov. 30, 1965. This method uses the desulphurizing action of uranium, which for example, in the case of Invar containing 36 percent nickel, is added in a low enough proportion for the coefficient of linear expansion of the alloy not to be affected.

It is also known to use a filler metal for welding which contains manganese and titanium so that several percent of each of these elements are added to the base ferro-nickel. But this has the serious disadvantage that it raises prohibitively the coefiicient of linear expansion of the alloy.

Whatever method is adopted to limit the effects of the presence of sulphur, the fact remains that the hot-shortness also depends on the state of crystallization of the metal. Coarse crystallization promotes lack of cohesion between the grains due to hot-stressing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I have now developed a process for obtaining modified ironnickel alloys with a high nickel content which are not subject to coarse crystallization in heat-affected zones during welding and which have substantially the same coefficient of thermal expansion as the unmodified alloys.

According to the process of the present invention an ironnickel alloy containing at least 30 percent nickel is obtained, said alloy having distributed therethrough a fine dispersion of 0 a nitride of at least one of the elements, vanadium, titanium,

. or transport of liquefied gases. The curves of FIG. 1 were obzirconium and niobium.

The nitride constitutes an insoluble phase in the alloy and in the form of a fine dispersion, it prevents or reduces recrystallization of the alloy on heating and thus prevents coarse crystallization of the alloy and the formation of coarse grains. The presence of the insoluble nitride dispersion also reduces embrittlement due to the presence of sulphur in the alloy.

The process according to the invention produces alloys which preferably contain from 0.02 to 0.1 percent of the nitride-forming elements. Vanadium is the preferred nitrideforming element.

It has also been found that the presence of the insoluble nitride phase, when its quantity is correctly adjusted, does not modify the coefficient of linear expansion of the alloys.

The nitride-forming elements which have a strong affinity for elements other than nitrogen, for example carbon, should be used in the process in a proportion such that sufficient nitride-forming element is present to combine with nitrogen to form nitride as well as forming compounds, such as carbides, with such other elements.

The nitrogen required to form the nitride will normally be that already present in the alloy, but if necessary nitrogen can be added before, during or after the addition of the nitrideforming elements to the melt.

Referring now to FIG. 2, two castings of 10 kg. each were made under vacuum, as two alloys A and B.

Alloy A is a known alloy while alloy B was processed to include a certain amount of zirconium in accordance with the present invention. The compositions of the two alloys when cast were:

Casting B differs from casting A only in the addition of zirconium, the other elements being equivalent.

The addition of zirconium is between the limits of 0.02 to 0.1 within the scope of the present invention.

The addition of zirconium does not change the properties of 5 expansion of the alloy. Alloy B, forged, annealed for 1 hour at 850 C. and tempered in oil, has a mean coefficient of expansionbetween l96 C. and ambient temperature, about 20, of l.38 l0' Alloy A under. the same conditions, was 1.23X10.

FIG. 2 shows as a function of temperature on the abscissa, the variation of elongation at break on the ordinant of a forged alloy annealed for 1 hour at l,100 C. and tempered in oil.

A comparison of curves A and B corresponding to alloys A and B show that the addition of zirconium (curve B) practically removes the drop in ductility observed for alloy A between 600 and l,000 C.

Parallel to the removal of the drop in ductility in alloy B is a considerable reduction in the size of the grain. The thermal treatment of 1 hour at 850 C. discussed above for measuring the coefiicient of expansion produced the following variations of grain ASTM:

Alloy A: ASTM 7 Alloy B: ASTM 11 The presence of zirconium nitride was identified in casting B by X-ray diffraction.

When alloy A was used for welded constructions, cracks appeared along the welds while for alloy B excellent welding took place, with no cracks in the welding zone, thus establishing the decrease of the drop in ductility as seen in FIG. 2.

0 SI S P Mn Ni Ti N2 Fe c 0.026 0.09 0.001 0.007 0.26 36.10 0.12 14 Ren mder.

The stretch of 700-800 C. is about 45 percent, slightly more than the stretch of iron-nickel alloys with 36 percent nickel and no addition of V, Ti, Zr, or Nb.

FIG. 3 shows the improvement caused by a small amount of titanium (0.10 percent) added to the alloy by the present process involving no change of expansion.

Iclairn:

l. A process for improving the welding characteristics of a weldable iron-nickel alloy consisting essentially of at least 30 percent nickel by weight which comprises introducing, during the preparation of the iron-nickel alloy, from 0.02 to 0.1 percent by weight of at least one of the elements, vanadium,

.titanium, zirconium and niobium, said element combining with nitrogen present in or introduced into the alloy to form the nitride thereof in fine dispersion in the alloy reducing recrystallization of the alloy on heating and reducing cracks in the alloy adjacent the weld.

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